Errors in the transmission of data can arise from many sources, such as adjacent channel and co-channel interference and undesired distortion in a sending device's transmitter and/or a receiving device's receiver. The result of the introduction of errors is a demodulated bit stream at the receiving communication device (data recipient) that is different from the bit stream that was modulated and transmitted by the sending communication device (data source). One method used to combat this problem is to introduce redundant information, thereby allowing the data recipient to deduce that an error has occurred and to correct the error. The redundant information may consist of additional data bits that are transmitted along with the information bits or may be embedded in binary codewords that are used to represent the information bits. In either case, a portion of the channel bandwidth must be allocated to provide this protection.
In addition to the above-listed sources of error in a transmission channel, mobile radio channels typically experience periodic, transient fading due to multipath propagation effects, which can cause bursts of errors in the demodulated bit stream. To help combat deep fades, interleavers are generally used along with redundant information to break up burst errors into manageable pieces. The interleavers redistribute the data bits contained in a block of data prior to transmission. The combination of adding redundancy to a block of data and interleaving the bits prior to transmission has proven to be a particularly effective method of combatting transmission errors since data containing a burst of errors is more likely to be correctly decoded after the burst has been broken up by the decoder interleaver.
It often happens that there are major differences in importance among the pieces of digital information designed to be transmitted in a block of data. When transporting compressed speech or audiovisual data, for example, often a subset of the data to be transmitted requires significant protection from channel errors, while the rest of the data requires less protection. An encoder that uniformly protects the data using an error protection code designed to protect the most sensitive bits adds excessive redundancy for the less sensitive bits and, as a result, occupies an excessive amount of bandwidth overhead. Such a code is suboptimal in terms of spectral efficiency.
Therefore, the need exists for a method and apparatus for improved error control that provides more error protection for more sensitive data and less error protection for less sentsitive data, thereby reducing the bandwidth consumed by the error control code and maximizing spectral efficiency.